In the following, the term document is used to refer to any valuable sheet of paper or other body printed thereon with ink containing a magnetic pigment having specified magnetic properties.
In the related prior art, the magnetic field detection system used for the detection and verification of a common document uses a multi-head (5-head) process shown in FIG. 1. A Document 11 is transported in a direction shown by the Arrow in FIG. 1. The Document 11 first encounters a high frequency erase head 15. The high frequency erase head 15 demagnetizes the magnetic ink on the Document. Next the Document 11 is passed over a non-saturation record head 16 where the Note 11 is exposed to a field that magnetizes the ink to approximately 80% of saturation. The Note is next passed over non-saturation read head 17, the non-saturation read head detecting a remanent moment resulting from the application of the non-saturation magnetic field. The Note 11 is next passed over a saturation record head 18 which provides a field which magnetizes the magnetic ink layer to saturation. Finally, the Note 11 is passed over a saturation read head 19 which detects the saturation remanence moment resulting from application of the saturation magnetic field. The operation of the prior art testing method can be understood by reference to FIG. 2. FIG. 2 is a plot of the magnetization of the material versus the magnetic field, commonly called the hysteresis loop. The Document 11 first has an erase field applied thereto which results in the magnetic layer having the magnetic properties identified by point 21 of FIG. 2. When a non-saturation (80%) magnetic field is applied to the Document 11, the magnetic layer has the properties associated with a point 22 relative to the hysteresis curve 20. When the Document 11 is moved from the vicinity of the non-saturation magnetic record head, the magnetic properties (remanence moment) assume the properties defined by point 23. This remanence moment defined by point 23 is measured by the non-saturation magnetic field read head 17. Document 11 is then subjected to a saturation magnetic field and the field in the magnetic layer is shown by point 24 of FIG. 2. Once removed from the saturation magnetic field record head 18, the remanence magnetization at point 25 is measured by the saturation magnetic field read head 19. The measurements of the remanence field at point 23 and point 25 are used to authenticate the Document 11.
The conventional magnetic ink detection apparatus described above requires an erase head. A conventional erase head consists of a gapped magnetic core wound with many turns of wire. The wire carries a high frequency A.C. field. When the Document passes in the vicinity of the gap in the core, the Document is subject to a large alternating polarity magnetic field. The magnetic field experienced by the Document decays to zero as the Document moves away from the gap. The large alternating field erases any preexisting magnetization that can be present in the Document. As a result of the application of the decaying large alternating field, the layer of the Document is at point 21 of FIG. 2.
In certain apparatus used in the scanning of the Federal Reserve Documents, a .+-.1 mm tolerance in the Document to erase head separation is established and the Documents are scanned at 10 m/sec. In order to erase the preexisting magnetic field effectively, the A.C. erase field must be very large and be modulated with a high frequency. This large, high frequency field generates a significant noise signal in the read heads 17 and 19 of FIG. 1. The result of the generated noise, is that the signal to noise ratio is lowered.
Because of the large, high frequency current flowing in the turns of wire of the erase head, an unacceptable power can be dissipated by the erase head and excess heating can occur.
A need has therefore been felt for apparatus and a procedure which can eliminate the need for the erase head in a Document magnetic ink detection system.